The present invention relates to an apparatus for purifying an exhaust gas from automobiles using gasoline engines.
Generally, gaseous pollutants discharged from automobiles are carbon monoxide (CO) and hydrocarbons (HC) due to incomplete combustion, and nitrogen oxides (NOx) formed by thermal dissociation depending upon a combustion temperature. These three pollutants each require contacting means for their purification on account of different generating mechanisms, and it is very difficult to abate these three pollutants at the same time. The simultaneous abatement also gives a considerable adverse effect upon driving performance, fuel cost, etc.
Under these circumstances, an apparatus using a single three-functional catalyst capable of oxidizing CO and HC and simultaneously reducing NO.sub.x has been so far regarded as important. The three-functional catalyst can have purification ratios each of CO and HC, and NO.sub.x of more than 90% approximately at the theoretical air-fuel ratio of 14.5. Therefore, if the air-fuel ratio of an air-fuel mixture to be supplied from a carburetor is controlled approximately to the theoretical air-fuel ratio under every driving conditions, all the gaseous pollutants can be purified at purification ratios of more than 90%.
A process has been proposed, as disclosed in U.S. Pat. No. 3,942,493, which comprises providing an oxygen concentration detector (the so-called O.sub.2 sensor) in an exhaust gas line to detect an oxygen concentration of the exhaust gas, and transmitting an output of the O.sub.2 sensor to an electromagnetic valve provided in an air bleed of carburetor through an amplification control means, thereby changing a flow rate of air passing through the air bleed and making an air-fuel mixture supplied from the carburetor approach the theoretical air-fuel ratio. However, the prior art process has several problems. For example, it is very difficult to control an air-fuel ratio of the air-fuel mixture supplied from the carburetor to a narrow range around the theoretical air-fuel ratio in every driving conditions, and a considerable increase in cost is inevitable for the control. Furthermore, a high precision sensor is necessary for exactly controlling the air-fuel ratio, but the currently available O.sub.2 sensors still have such problems as unevenness in product quality lots by lots, unevenness in their performances depending upon driving states, durability, reliability, etc.